Method and apparatus for recovering information from a videodisc

ABSTRACT

A system for recovering information from a videodisc and performing special functions which enhance the operational character of the system. To increase the reliability of landing at or near a target track, provisions are made to search for a track adjacent a target track whose track identifier has been obliterated. Further, a method and means for stepping forward or reverse one field at a time is disclosed. Another aspect of the invention concerns a capability to respond to a random command instruction resulting in subsequent functioning of the system in a random manner. Yet a further aspect of the invention is related to controlled jumping of one or more tracks during vertical blanking time to produce unique forward or backward motions; multiples of playing speed, both forward and reverse, and visual special effects can be realized using the multiple track jumping feature. Finally, a method and means are described which permit synchronous transmission of video from a videodisc to an external using device.

FIELD OF THE INVENTION

This invention relates generally to systems for recovering informationfrom video recording discs, and more particularly to a system that movesan information recovery device rapidly, relative to a plurality ofinformation tracks on a videodisc, toward a selected target track on thedisc to recover information recorded on the target track. When desired,special functions are subsequently performed which enhance theoperational character of the system.

DESCRIPTION OF THE PRIOR ART

An example of a system of this particular type is described in aco-pending application for U.S. patent Ser. No. 295,629, filed Aug. 24,1981, and entitled "Method and Apparatus for Information Retrieval froman Optically Readable Storage Medium", now U.S. Pat. No. 4,375,091, andassigned to the assignee of the present application. The disclosedapparatus directs a reading beam of light at a selected one of aplurality of spiraling and substantially circular information tracks orconcentric circular information tracks on a rotatable recording disc.The intensity of the beam is modulated by the recorded information whichincludes a unique address signal for each track, and the apparatusdetects the modulated beam to produce a playback signal indicative ofthe recorded information.

The known apparatus includes means for selecting a particular targettrack to be scanned, together with a carriage motor and a movable mirrorfor moving the reading beam radially toward the target track with coarseand fine movements, respectively. The apparatus further includes meansfor monitoring the playback signal to detect periodically the trackidentifying code, or address, of the track currently being scanned. Suchcurrent address is compared to the target track identifying code, oraddress, and the apparatus applies prescribed sequential drive signalsto the carriage motor, depending on the remaining distance to the targettrack. The speed of the carriage motor is stepped successivelydownwardly as the reading beam reaches predetermined distance thresholdsduring carriage translation. During the last stage of carriagetranslation, the movable mirror increments the beam radially by onetrack spacing during each revolution of the disc, thereby "playing" intothe target track.

Although the information recovery system described above has proveneffective in rapidly moving a reading beam toward a selected targettrack to recover information recorded thereon, the system does notoperate satisfactorily whenever the unique address code for the targettrack is obliterated or is otherwise not reliably detectable orreadable. In prior art disc players of the type described above, in theevent that the track identifying code for the target track isobliterated, a "match" of addresses is not possible, and the playereither searches to the end of the disc information and idles there or isinstructed, under microprocessor command control built into the machine,to perform the search again. In the event that the search functionfailed on the first pass because it did not receive a proper identifyingaddress code, it is possible that a second search pass will result insuccess in landing on the target track. This could happen, for example,if the read head was passing laterally of the disc at too fast a rate atthe particular time that the address codes contained in the verticalinterval portion of the recorded signal was passing by. In such a case,making 2, 3, or even more passes in a search mode could result in anultimately successful target track "find".

However, no number of retries will result in a successful "find" if theaddress code for the target track is obliterated due to a surface defecton the disc, or is not readable for any one of a variety of otherreasons which can be attributed to the disc making process, the tape todisc transfer process, the tape recording process, and even the encodingequipment used to insert the address codes on the mastering tape in thefirst instance. In any event, with an obliterated address code for aparticular target track, the aforementioned repeated search functioningof the player would be an exercise in futility, since a "match" ofaddresses would never occur.

It is further to be noted at this point that performing repeatedsearches in prior art machines required the same amount of time toperform each unsuccessful search, since the search is usually tried forsome fixed period of time to locate the designated target track beforegiving up as unsuccessful. Upon initiation of any search, the timeperiod was set to some fixed constant greater than the longest possiblesearch across the whole disc. In prior art players, this process ofunsuccessful searching for a selected target track could span eightseconds of real time. Accordingly, 2 or 3 passes would be aninordinately long length of time for a user to wait for the selectedtrack, even assuming that it could be found eventually. In the eventthat it was never found, the user would be left with uncertainty as towhether or not the track will eventually be found, and if it is notfound, the user must stop the searching process and refigure a newapproach to get close to the program material he is desiring to view. Inthis connection, another U.S. application Ser. No. 316,021, filed Oct.28, 1981 and entitled "Method and Apparatus for Recovering Informationfrom a Selected Track on a Record Disc", now abandoned in favor of acontinuation-in-part application Ser. No. 333,236 filed Dec. 21, 1981,also assigned to the assignee of the present application, discloses animproved searching method and apparatus in which the radial velocity ofthe search path varies inversely as some function of the differencebetween the selected target track address and the current address beingdetected while scanning. Additionally, the latter mentioned U.S.application illustrates an improved search function by permitting themachine to search for the target track from both directions and withchanging velocity depending upon distance from the target track. Bypermitting the player to search in both directions and at variablespeeds, the total search time for finding the target track issubstantially reduced. This can be appreciated from the fact that, oncethe read head passes, relatively speaking, the target track, the playermerely reverses the diredtion of motion of the read head relative to thedisc to again search for the target track by detecting its identifyingaddress code. Thus, since the read head is already in the vicinity ofthe target track when its scan motion is reversed, the length of timenecessary to reach the target track in the reverse direction issubstantially reduced, and this analogy can be carried on indefinitelyeach time the target track is passed during the search mode. Again,however, in the event that the target address code is never detectedbecause it has been obliterated for some reason, the player willcontinue its search function indefinitely in the vicinity of the targettrack. This assumes, of course, that the reader of the disc player isable to retrieve and identify track identifying codes in the vicinity ofthe target track so that the scan control electronics of the player candetect that the read head has overshot its target.

Whenever the address code of a selected target track is difficult toread or not readable at all, it can be appreciated that prior art discplayers suffer from the inability to eventually locate that track or atleast to locate a part of the recorded information reasonably close tothat originally desired, and additionally such disc players suffer fromthe inability to stop searching for a target track when it is never tobe found. There is thus a need for a system that can provide selectionof an information track relatively close to the target track even whenthe target track identifying code is obliterated, and upon failing thatto at least cease searching for an unidentifiable track address after aprescribed length of time has passed in the search mode. There is also aneed for a playback system which will, to the extent possible, arrive ata target track even though the target track address identifying code hasbeen obliterated.

Another failing of the prior art closely related to the above-mentionedplayer operational functions is the inability to freeze frame a selectedinformation track when the white flag or frame address code informationis obliterated. In this connection, in the known machines, a freezeframe or still frame as it is known is obtained by repeating twoconsecutive fields one.of which is identified by having a selectedhorizontal line of information in the vertical interval to represent anall white signal for that line, thereby defining a "white flag".Appropriate detecting circuits are employed to recognize the presence ofa white flag, and upon such detection cause the tracking mirror toreposition the beam, during vertical interval, one frame backwards so asto repeat that frame indefinitely. Like the search feature of theplayer, however, if the white flag is obliterated, the typical playerdoes not use sufficient other information to effect freeze frameoperation, and the player will latch onto the next readable white flagand freeze frame the frame associated therewith and not the selectedone. Accordingly, there is a need for a player system to be able tofreeze frame without the presence of a white flag or the accuraterecognition of a selected frame identifying code.

Another shortcoming of the prior art concerns the fact that the programmaterial arranged on a videodisc was rather consecutively structuredwhen the disc was made. In other words, information to be communicatedto the user was organized in a rather straight forward cascade ofinformation segments. When, for example, appropriate instruction in alearning situation has been given, a series of questions would bepresented to the user to test his learning ability, comprehension, andretention of facts. Accordingly, a series of questions would bedisplayed on the screen or a similar series of questions would bepresented audibly to the user. The problem with such a straight forwardapproach is that the same disc provides the same questions in the sameorder each time the user reviews the material. To create a widerinterest in the learning process, and to prevent cheating by one studentuser having learned the order of answers from another student user, aswell as for many other related reasons, there is a need to provide arandomness to the manner in which questions to tests are given. Ofcourse, there would also be a need for such random presentation ofinformation even out of a test-giving situation to create additionalinterest for the user and enhance the overall value of recordedprograms. The present invention fulfills this need by providing a discplayer capable of random branching among the information tracks so as topresent a different resulting program each time the disc is played.

Yet another shortcoming of the prior art concerns the limited ability ofthe read beam to jump multiple tracks in a synchronized manner. It isknown, of course, to perform high speed scans, both forward and reverse,which result in a rather random crossing of tracks by the read beam asthe read head and disc move radially of one another at a rather highspeed. The picture viewed on a monitor in this scan mode is severelysegmented with noise streaks, and the sound is, of course,unintelligible. In the prior art machines, there is the ability tosearch for a particular frame and freeze frame or begin play at theselected frame as well as the ability to jump backwards one frame torepeat a frame in a freeze frame;presentation. However, it is desirableto be able to play the program material at faster than normal speeds,both forward and reverse, and yet be able to view a complete fullysynchronized picture while doing so. At speeds close to normal playingspeed, it may also be desirable to listen to the sound track, whetherforward or reverse, for purposes of establishing reference points andthe like. Accordingly, there is a need to provide a playback systemwhich incorporates the ability to jump forward or jump reverse aspecified number of frames while yet maintaining full synchronization ofthe signal viewed on a monitor.

Another problem closely associated with synchronizing effects with thewhite flag information contained in the vertical interval of therecorded program on the disc, as mentioned by example earlier, is theinability to transfer information from the disc to an external stationin synchronization with a command to transmit issued from the externalstation. In other words, while a disc player provides full internalsynchronization of its own recovered signal to a monitor, it is oftenthat external equipment receiving information does not receive suchinformation in full synchronization with its "fetch" commands.Accordingly, there is a need to provide a playback system which canreceive a "fetch" command from an external station, wait an appropriatelength of time regardless of the relative time between receipt of the"fetch" command and the timing of the information being played by thedisc player, and output the required information in full synchronizationand compatibility with the external station.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a method and apparatus for adisc player which has the ability to eventually locate a target track,or at least to locate a part of the recorded information reasonablyclose to the target track, in the event that the information on the discidentifying the track location has been obliterated. In a preferredembodiment, a further object of the invention is to stop searching for atarget track when it is never to be found.

Such an improved search feature for a disc player is realized in thepresent invention by the provision of a "variable landing pad". As usedin this description, the term variable landing pad concerns the desireto search for a particular frame on the disc, and in the event thateither the frame number or white flag identifier is obliterated at thedesired location, to be able to find another adjacent, or close, frameat which the read head of the disc player stops to complete a searchfunction. It is often the case that a particular frame searched for isnot necessarily exclusive insofar as the user is concerned, since one ormore frames on either side of the selected frame would be just asvaluable for the user.

Of course, when viewing a catalog of items wherein each frame is unique,a variable landing pad in the sense described above is not desirable,since only the particular frame searched for is of value to the user.However, by incorporating the landing pad search feature so that a frameclose to the selected frame is found, it is possible for the player tothen count up or count back a particular number of frames to arrive atthe desired, originally selected, frame, even though the frame number orwhite flag information identifying that selected frame is obliterated.

In the event that exactly locating a specific frame number is not ofextreme importance to the user, such that perhaps as many as 30 frameson either side of the selected frame is an acceptable landing point forthe read head, then upon failure to find a selected frame, the searchdirection can be reversed and a retry to find the same frame would beinitiated. After failure to find the selected frame in a predeterminedlength of time, the search mechanism would try to find an adjacent framewith the same kind of back and forth searching motion for a secondpredetermined length of time, and in the event of a failure to find theadjacent frame, to proceed in like manner to locate the next adjacentframe, and so forth until an acceptable frame number within a prescribeddistance from the originally selected frame number is found. Of course,in the event that no closely related frame is found, the read head canbe instructed to merely stop after a prescribed number of retries atwhatever frame it is at at the time, or alternatively can be instructedto stop after a predetermined fixed length of time. The advantage of the"retry" function just described is that, since the frame number searchedfor originally is approached on the first try, each retry can take lesssearch time, so that the ultimate frame landed on will be reached in asubstantially shorter time than with a full time period search retryfunction as has been done in the prior art.

An extension of the variable landing pad feature concerns the ability totake alternative action after a fixed search time has passed or after apredetermined number of retries has taken place as indicated above. Theoption discussed above was to merely stop and play at such time.Alternatively, if the disc had the program material recorded in two ormore bands on the disc (obviously good for relatively short programsonly), after failure to find a target track in one band, i.e. afterseveral "retries", the player can search for the duplicated target framein another band by incrementing the target frame address code by aconstant. Again, if the updated target track is not found on the firsttry, the updated target track plus one would be searched for in a mannerdescribed above. Note should be made of the fact that the automaticincrementing to another information band upon failure to find a selectedtarget track in the first band is covered by another applicationentitled "Banded and Interleaved Videodisc Format", U.S. applicationSer. No. 327,321, filed Dec. 4, 1981 and assigned to the assignee of thepresent application. The incrementing by one frame address and"retrying" are aspects of the present invention, however.

Another object of the invention closely related to the above-mentionedvariable landing pad function is to provide a stop motion function, theability to freeze frame a selected information track, when the whiteflag or frame address code information of the selected frame to beviewed is obliterated. Further, independent of the fact that a pictureframe is comprised of two (and sometimes three) adjacent and consecutivefields, the present invention provides the ability to recover andrecognize the vertical synchronization pulses contained in the recordedinformation on the disc, generate a stop-motion enable signal, andeffectuate stop-motion by utilizing the next vertical synchronizationpulse in time after generating the enable signal to pulse the trackingservo of the disc player one track in reverse, and for additionallypulsing the tracking servo one track in reverse for every predeterminedmultiple of two vertical synchronization pulses detected. In a preferredembodiment, the predetermined multiple is one, i.e. the tracking servois pulsed on every other vertical sync pulse.

In effecting stop-motion by the manner just described, the first fielddisplayed in each group of two fields in the stop-motion mode could bean even field or an odd field, since the jump-back of the tracking servois initiated by the next vertical sync pulse occurring in time afterbeing enabled by an arbitrarily positioned, usually manually inserted,enabling signal. The present invention permits playing from thestop-motion position by producing a one-field-step-advance orone-field-step-reverse motion while maintaining stop-motion playback ofthe information track after being stepped. This is accomplished byeither disabling the effect of the next vertical synchronization pulsein time after initiation of the field-step enable signal or bypermitting an extra vertical sync pulse to "kick" the tracking servoafter the field-step mode signal is generated.

Another object of the invention is to provide a disc player and methodfor generating a random list of instructions for the disc player, sothat random information can be presented to the user when such randominstructions are encountered on playback of the disc. For example, thedisc can contain at one point in a recorded program a series of tenquestions occupying ten different tracks on the disc. If each time theuser was quizzed he or she was presented with the same questions, thetest would quickly become boring. Furthermore, if it was the intent toask only a few questions, it would be easy for a person taking the quizto get answers from a previous user, and this would destroy the value ofthe quiz given. The random command feature of the present inventionpermits branching out at a particlar point in the program material toone of a group of questions in rather random form. A random numbergenerator is provided, and depending upon the number arrived atrandomly, a series of questions, for example three randomly picked fromthe set of ten, would be presented to the viewer. Upon replaying thedisc and arriving at the same point of decision to present questions,the random effect would result in presenting the viewer. with adifferent set of three questions from the available ten. Additionally,even each random question, after it is presented, could branch toanother random question or the next in line of a series of questions. Inthe former case, the questions would indeed be randomly selectedindividually from a large group, and in the latter case a new group ofquestions would be selected from possibly a large number of questiongroups. Of course, other practical uses for the random command featureaccording to this object of the invention are realizable, and the aboveare merely examples.

Yet another object of the invention is to provide a method and apparatusfor accomplishing multiple track jumps during vertical interval time andin a synchronized manner in order to produce a stable picture withoutvisible track jumping noise which occurs when tracks are jumped outsideof the vertical blanking times. This object of the invention is realizedby playing back a selected track on a rotating disc while following thetrack with a read head, and selectively jumping across a predeterminednumber of tracks by incrementally moving the read head radially of thedisc during the vertical interval of the recovered video signal, andplaying at least one field of the recovered video signal after jumping.With this effect, it is possible to play the program material at fasterthan normal speeds, both forward and reverse, and yet be able to view a"complete" fully synchronized picture while doing so.

Finally, a further object of the invention is to provide a method andmeans for transferring information from a videodisc to an externalstation in synchronization. Basically, this feature of the inventionpermits the successful identification of a particular field of aparticular frame searched for on the disc. One of the problemsassociated with the prior art technology in the videodisc field concernsthe need for precisely identifying the first or second (and sometimesthird) field of a frame in order that synchronization with externalequipment can be effected.

For example, if program material is to be taken from the disc downstreamof the lead-in portion, it is necessary to identify when that programmaterial is to be taken off a disc, and to appropriately set up atransmit enable signal so that the occurrence of the next vertical syncpulse will signify the beginning of the program material to be loadedinto the external equipment from the disc. This particular feature ofthe invention is termed "white flag wait", an explanation of the termbeing presented later in this description.

Another example of the use of the white flag wait feature is that ofstop-motion audio where digitized audio is to be taken from a disc andplayed out in real time while the disc is still framing, and in such acase, it is necessary to transmit the digitized audio information atprecisely the appropriate time that it occurs on the videodisc relativeto the played-back program. Thus, if the digitized audio is to begin onthe first field of a selected frame, it is necessary to generate atransmit signal on the second (or last) field of the previous frame sothat the information contained in the next field (and perhaps subsequentfields) is available for being taken off the disc at the exact time thatthe external equipment is ready to receive it. Without proper fieldidentification, a transmit command could be premature by one field orpossibly delayed by one field so that the information transferred to theexternal equipment is picked up too early or in the middle of theinformation to be transmitted. The white flag wait feature of theinvention is effective to reliably identify the second (or last) fieldof the frame preceding the frame from which the desired information isto be taken. Of course, in the event that the information to be takenfrom a disc begins on a second field of a frame, a slight modificationof the apparatus can be made to accommodate this altered form of theinvention.

In a playback system wherein the video information retrieved from thedisc contains in part a sequence of vertical synchronization pulsesoccurring at a field rate, wherein a complete picture frame is definedby two fields, and wherein a white flag identifies the beginning of thefirst field of a two-field-per-frame set, the invention is implementedby searching and locating a designated frame next preceding in time thebeginning of the intelligence information to be extracted from the disc,identifying the next white flag signal in time, jumping back two fieldsupon identifying same, and playing the two fields in a forward playmode. Then, utilizing the next vertical synchronization pulse in time,after the jumping back two fields and during playing of the two fields,a transmit enable pulse is generated. The system utilizes the transmitenable pulse to generate a data flow enabling gate at the coincidence ofthe transmit enable pulse and the next white flag signal occurring intime. This results in the gating of the intelligence information to theexternal receiving device with the data flow enable gate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of the components of a videodiscplayer which function to carry out the objects of the invention;

FIG. 2 is a more detailed block diagram of the portion of FIG. 1concerning the tracking and track jumping characteristics of theinvention;

FIG. 3 is an illustration of the relationship between an open looptracking error signal and the radial cross-section of tracks on avideodisc;

FIG. 4 is a signal flow diagram used in carrying out the basic searchalgorithm as particularly used with the variable landing pad aspect ofthe invention;

FIG. 5 is a signal flow diagram of the portion of the function generatorof FIG. 1 used in carrying out the objects of the invention concerningthe variable landing pad search mode;

FIG. 6 is a graph illustrating the manner in which a read beam functionsto search for a target track in the variable landing pad mode of theinvention;

FIG. 7 is a block diagram showing the details of the components of avideodisc player involved in performing a stop-motion effect;

FIG. 8 is a pictorial representation of the path taken by a read beamwhen stepping one field at a time in reverse direction and then stillframing;

FIG. 9 is a pictorial representation of the path taken by a read beam ofa videodisc player when stepping one field at a time in the forwarddirection and then still framing;

FIG. 10 is a preferred form for the circuitry involved in carrying outthe step forward and step reverse one field at a time function;

FIG. 11 is a timing chart showing the various signal relationshipsinvolved in carrying out the field step function of the disc player;

FIG. 12 is a block diagram of the circuitry needed to carry out therandom-command function of the present invention;

FIG. 13 is a pictorial representation illustrating one example of theuse of the random-command feature of the invention;

FIG. 14 illustrates the radial path taken by a read head and the visualeffect on a monitor screen of the effects of a special arrangement ofmultiple track jumping with full synchronization;

FIG. 15 is a block diagram of the synchronous jump electronicsassociated with the multiple track jumping feature of the invention;

FIG. 16 is a table showing the direction and number of tracks jumped toperform various forward and reverse speeds for the videodisc player;

FIG. 17 is a timing chart showing the relationships between the variouswaveforms necessary to perform a white flag wait function according tothe invention; and

FIG. 18 is a block diagram showing the interconnections foraccomplishing the white flag wait feature of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and particularly to FIG. 1, there isshown apparatus for moving a reading beam of light 11 in a radialdirection relative to a rotating recording disc 13, to recoverinformation from a selected target track on the disc. The disc includesa plurality of closely spaced, circular and concentric recording tracks,and each track records a video signal representing one or more videoframes, with a unique frame or track address signal located in eachvertical interval (i.e., two address signals per frame).

The apparatus includes a spindle motor 15 for rotating the recordingdisc 13 at a prescribed constant angular velocity, and an optical system17 and an objective lens 19 for focusing the reading beam 11 onto aselected track of the rotating disc. The reading beam is reflected bythe disc to produce a reflected beam 21 having an intensity that ismodulated in accordance with the recorded information. The objectivelens and optical system direct this reflected beam to a detector 23which detects and demodulates the modulated intensity of the beam toproduce a baseband video signal corresponding to the recordedinformation. This video signal is coupled over line 25 to both a monitor27 and an address recovery and estimator circuit 29. The monitorprovides a real-time display of the video signal recovered from thetarget track, and the address recovery circuit detects the addresssignals in the successive vertical intervals of the video signal, usingconventional techniques. The address recovery circuit then updates anaddress register 30 with each detected track address signal.

The apparatus further includes a coarse positioning system and a finepositioning system for controllably moving the reading beam 11 radiallyrelative to the disc 13, toward the selected target track on the disc.The coarse positioning system includes a carriage motor 31 andappropriate gearing 33 for moving the beam at a selected one of tworelatively high radial velocities (e.g., 100 and 500 tracks per discrevolution). The fine positioning system includes a movable mirror (notshown) located in the optical system 17 for contrallably adjusting thebeam's point of impingement on the disc over a relatively small range(e.g., about 50 tracks in either direction).

When a user desires to recover information recorded on a selected targettrack on the disc 13, he or she inputs a special target address codesignal on line 35, indicating the target track's address. A functiongenerator 37 compares this target address signal with the address signalcurrently being stored by the address register 30. In accordance with aprescribed algorithm described in the aforementioned U.S. patentapplication Ser. No. 316,021, the function generator determines theradial separation between the current track and the target track, andoutputs appropriate control signals to controllably drive the carriagemotor 31 and the movable mirror of the optical system 17 so as to movethe reading beam 11 toward the target track. These control signals aresequenced such that the beam reaches the target track in a timesubstantially less than that achieved by prior apparatus of this kind.

The carriage motor 31 is driven at a prescribed velocity (or sequence ofvelocities) until the reading beam 11 has been moved to the vicinity ofthe target track, i.e. to within a prescribed number of tracks of thetarget track, after which the movable mirror of the optical system 17 isconditioned to incrementally jump the beam from one track to the next aplurality of times during each half revolution of the disc 13.

More particularly, the coarse positioning system further includes atrack scan driver 39 and a carriage motor tachometer 41 for controllablydriving the carriage motor 31 in the prescribed fashion. The functiongenerator 37 outputs a plurality of velocity commands for coupling overlines 43a-d to the track scan driver, which, in turn, controllablyadjusts a dc drive signal coupled over line 45 to the carriage motor.The tachometer feeds back to the track scan driver over line 47 acarriage tachometer signal indicating the carriage motor's angularvelocity, to enhance control of that velocity. Also, a dc tracking errorsignal is coupled to the track scan driver over line 49 to controllablymove the carriage motor so as to reduce any steady state deflection ofthe movable mirror of the optical system 17.

Referring again to FIG. 1, the fine positioning system further includesa tracking servo in the form of a tracking controller 67 for producing aradial correction signal for coupling over line 69 to the movable mirrorof the optical system 17. Depending on the operating mode of theapparatus, this signal either maintains the reading beam 11 aligned witha selected target track, or incrementally jumps the beam from.track totrack while approaching the target track. The tracking controllerreceives a plurality of track jump commands supplied on lines 71, 73 and75 from the function generator 37, along with a tracking error signalsupplied on line 77 from the detector 23.

When the apparatus is operating in a mode in which the reading beam 11is to be maintained in alingment with a selected track, the trackingcontroller 67 merely amplifies the tracking error signal and couples itdirectly to the movable mirror of the optical system 17, to form aconventional closed loop tracking servo for controllably aligning thebeam with the track. On the other hand, when the apparatus is in asearch mode in which the beam is to be moved incrementally from track totrack, the tracking error signal is uncoupled from the movable mirror,and a prescribed sequence of pulses is coupled in its place.

The tracking controller 67 is depicted in greater detail in FIG. 2. Itincludes a disable switch circuit 79, an amplifier 81, and a powerdriver 83, for amplifying the tracking error signal supplied on line 77and outputting it as the radial correction signal for coupling over line69 to controllably position the movable mirror of the optical system 17(FIG. 1). The tracking error signal is coupled through the disablecircuit 79 at all times except during the search mode of operation orwhen jumping multiple tracks in a jump mode to be discussed later. Theoutput of the disable circuit is coupled over line 85 to the negativeinput terminal of the amplifier 81, and the ouptut of the amplifier is,in turn, coupled over line 87 to the power driver 83, which outputs theradial correction signal. The signal output on line 85 by the disablecircuit is also coupled to a low-pass filter 89 to produce the dctracking; error signal for coupling on line 49 to the track scan driver39 (FIG. 1).

At the beginning stage of each search mode of operation in which thecarriage motor 31 moves the reading beam 11 rapidly toward a targettrack, the tracking error signal experiences wide variations in 1evel asthe beam crosses the successive tracks. With reference to FIG. 3, whichis a fragmentary cross-sectional view of the disc 13 showing threerecording tracks along a disc radius, it will be observed that theopen-loop tracking error signal is a large amplitude ac signal having alevel of zero at the center line 91 of each track. The disable circuit79 uncouples the tracking error signal from the amplifier 81 at thistime, to ensure that the apparatus does not attempt to controllablyalign the reading beam 11 with any track as it moves radially toward thetarget track.

In the search mode of operation, the coarse positioning system, whichincludes the carriage motor 31, operates whenever the distance betweenthe target track and the present track exceeds a prescribed threshold,and the fine positioning system, which includes the movable mirror ofthe optical system 17, operates whenever this distance does not exceedthe threshold. When the coarse positioning system is operating, atracking disable command is coupled over line 71 from the functiongenerator 37 to the tracking controller 67. This signal is coupledthrough an OR gate 93, and in turn over line 95 to the disable circuit79, to uncouple the tracking error signal from the amplifier 81. Theradial correction signal output by the tracking controller 67 on line 69therefore has a level of zero, and the movable mirror remainsstationary.

After the reading beam has been moved to a position within a prescribednumber of tracks of the target track, the function generator 37 (FIG. 1)no longer outputs velocity commands to the track scan driver 39, and thecarriage motor 31 is no longer driven at a relatively high speed. Aprescribed time delay thereafter, the function generator terminatesthe - tracking disable command previously coupled over line 71 to thetracking controller 67, so that the tracking error signal is againcoupled through the tracking controller to form the tracking servo loopfor controllably aligning the reading beam 11 with whatever recordingtrack the beam arrives at. Thereafter, the tracking controller outputs aprescribed sequence of pulses to jump the reading beam incrementallyfrom track to track until it reaches the target track.

To effect the incremental jumping, the tracking controller 67 includes akick generator 97, a zero crossing detector 99, a jumps-down counter101, and a flip-flop 103. When the incremental jumping is to beinitiated, a binary code indicating the number of tracks (e.g., 11tracks) to be jumped during the next half revolution of the disc 13 issupplied on lines 73 from the function generator 37 and entered into thejumps-down counter. Simultaneously, a jump command signal supplied online 75 from the function generator is coupled to the set direct inputterminal of the flip-flop. This sets the Q output signal into thelogical "1" state, and this signal is coupled over line 105 to the ORgate 93, and in turn over line 95 to the disable circuit 79, to open thetracking servo loop.

The Q output signal of the flip-flop 103 is coupled over line 107 to thekick generator 97, which responds by outputting a single pulse signalfor coupling over line 109 to the positive input terminal of theamplifier 81. This pulse signal is coupled through the amplifier andpower driver 83 to the movable mirror of the optical system 17, toaccelerate the reading beam 11 in the direction of the target track.

After the reading beam has been accelerated in the direction of thetarget track by the kick generator 97, the zero crossing detector 99monitors the open loop tracking error signal (FIG. 3b) supplied on line77 and outputs a clock pulse each time it detects a track crossing bythe beam. These successive clock pulses are coupled over line 111 to theclock terminal of the jumps-down counter 101, to decrement the storedcount, accordingly. When the count reaches zero, the counter outputs areset pulse for coupling over line 113 to the reset direct terminal ofthe flip-flop 103.

The reset pulse coupled over line 113 to the reset direct terminal ofthe flip-flop 103 returns the Q output signal to the logical "1" state,which triggers the kick generator 97 to output a pulse of oppositepolarity to that of the original pulse, thereby decelerating the movablemirror. The reset pulse simultaneously returns the Q output signal ofthe flip-flop to the logical "0" state, so that the tracking servo loopis no longer disabled by the disable circuit 79 and the loop can againfunction to controllably align the reading beam 11 with the track thenbeing scanned. During this time, the dc tracking error signal is coupledon line 49 to the track scan driver 39, to controllably move thecarriage motor 31 so as to reduce the deflection of the movable mirror.

For a low number of tracks to jump, e.g., ten or less, the reading beam11 preferably traverses the prescribed number of tracks during thevertical interval. Since the monitor screen is then blanked, a noiselessjump will have been effectuated.

The kick generator 97 can include two monostable multivibrator orone-shot circuits, one triggered by a positive-going transition and theother by a negative-going transition. The kick generator can furtherinclude appropriate gating circuits to ensure that the successive pulsesit produces have the correct polarity to move the reading beam 11 in thedirection of the target track. These gating circuits are responsive tothe forward and reverse direction commands supplied on lines 43c and43d, respectively. Examples of other kick generator circuits that can beappropriately modified to provide the recited functions are disclosed ina copending application assigned to the assignee of the presentapplication, Ser. No. 130,904, filed Mar. 17, 1980, and entitled"Tracking System For Player," and in the patents cited in thatapplication.

In an alternative embodiment, the tracking controller 67 radiallyaccelerates and decelerates the reading beam 11 such that it moves byjust one track spacing each time. This type of movement is disclosed inthe copending application, Ser. No. 130,904, referred to in thepreceding paragraph.

After the reading beam 11 finally reaches the target track, theapparatus can operate, for example, in a stop-motion mode, to scan thetrack repeatedly and display the recovered video signal. If thesuccessive tracks are arranged in a spiral pattern, the apparatus mustjump the beam backward by one track spacing during each disc revolution,preferably during a vertical interval. Suitable apparatus for effectingsuch track jumping is disclosed in the aforementioned copendingapplication, Ser. No. 130,904, which is incorporated by reference, andin the patents cited in that application.

Variable Landing Pad

FIG. 4 illustrates the search algorithm in the form of a signal flowchart where a start command 121 initiates a search mode in the playerand outputs a "success" signal if the present position (P) is equal tothe target position (T) in step 123 which determines the relationshipbetween P and T. It should be recalled here that the function generator37 (FIG. 1) compares a target address input on line 35 with the presentposition address from address register 30, and the basic search functionis performed in a manner similar to that of the aforementioned U.S.patent application No. 316,021.

Depending upon which direction from present position the target track islocated when the search is initiated, in the event that the address codeor white flag information at the target address is obliterated, either asearch forward step 125 or a search reverse step 127 will drive thesearching elements, carriage motor 31 and tracking mirror of opticalsystem 17, depending upon whether P is less than T or P is greater thanT, respectively. The search drive step 129 will continue to drive thesearching elements toward the target track at a drive speed which is afunction of the distance left to travel to the target track, and if notfound will reverse the direction of the search and "look" again, andthis procedure will continue indefinitely in the prior art machines.However, the present invention incorporates a step 131 which asks thequestion "has time elapsed!", and if a prescribed time limit has notpassed, the "no" output of step 31 permits a "retry" of the search byinitiating another search command at the input to step 123. If theprescribed time has elapsed, then a "yes" output indicates failure tohave found the target track searched for.

Turning now to FIG. 5, a more detailed description of the variablelanding pad search feature of the function generator 37 will now bedescribed. The search algorithm 157 from FIG. 4 has been incorporatedinto the more detailed diagram of FIG. 5, and again a "start search"command is given at step 151. The parameters listed in step 153 are thenestablished, where T represents the target track, number being searched,R represents the number of retries to perform, and I indicates whetheror not the target track identification number should be incremented oneach retry. T_(o) represents the initial (desirable) target tracknumber, and as mentioned earlier, P represents the track number of thetrack being played at the present position. With these parameters set, a"set time out" step 155 establishes a first time within which asuccessful search must be made or alternative action will be taken. Inthe instant case, and by way of example, step 155 sets the time limitfor successfully finding the target track at 4.25 seconds.

The "success" result of step 157 is ultimately utilized to affect thestop search step 161 at which time the next instruction in theinstruction set is selected. However, in the "success" path is step 159which queries whether or not P equals T_(o). That is, if the tracknumber of the present position is equal to the initial target tracknumber, then the search is stopped and the next instruction selected asindicated. However, as will be seen later, a track other than theinitial target track may be arrived at, in which case step 163 iseffective to operate the tracking controller 67 in a manner to jump(P-T_(o)) tracks in the direction toward T_(o). In this manner, eventhough the track number finally searched for and found does not equalthe initial track number, the player nevertheless arrives at the initialtarget track by subsequently jumping the proper number of tracksrepresenting the difference between the track number of the "found"track and the number of the initial target track.

A track other than the desirable target track may be "found" in thefollowing manner. If the identifying code or white flag is obliteratedor otherwise not readable at the target track position, a failure willresult from step 157, and step 165 will test to determine if there areany retries. A retry is the reversing of the scanning direction afterthe tracking controller can identify that the difference between thepresent position track number and the target position track number isincreasing, indicating that a successful search was not found as theread beam of the player scans toward the target track and beyond. Aprescribed number of retries has already been set in step 153 asindicated earlier. Assuming that a failure to find the target trackoccurred on the first try, and assuming that the number of retries isgreater than zero, a "yes" output from step 165 causes a decrement ofthe R register in step 167 followed by the setting of a new "time-out"value of 1.5 seconds in step 169. If I=1, the target track number justsearched for will be incremented by one in step 173, and the searchalgorithm step 157 will now perform a further search for a new targettrack number equalling the previous target track number incremented byone. If, on the other hand, I=0, the target track number is notincremented, and the search step 157 will again search for the sametrack number as in the previous search step. This process will continuefor so long as there are retries left according to step 165 and time hasnot elapsed in step 131. Since surface blemishes on a disc may spanseveral tracks, it is possible that several adjacent adress codes and/orwhite flags of adjacent tracks will be obliterated. After some number ofretries, however, a successful find of the incremented target track mayresult in a stop search step 161 as previously indicated. Of course, thesearch function can look for the same target track as previouslysearched if the target track number is not incremented in step 171.Since a second try for the same target track number can produce success,it is not always necessary to increment the track number in order toobtain success in finding the track searched for.

It should be noted that the initial time out for the search algorithmwas 4.25 seconds, and this is considered necessary, since the targettrack may have been a substantial distance from the initial positionwhen the start search command was given. However, even if the targettrack is not found within the first prescribed time-out period, the readbeam should be in the vicinity of the target track, and therefore thesecond time-out length of time is set at 1.5 seconds in step 169. Theamount of time set in steps 155 and 169 are purely a matter of choice,and for the present preferred embodiment of the invention, each retryafter the first will be allowed 1.5 seconds of time. A limitation,however, on the total length of time allowed for searching is the timeset in step 131 which is an overall time for search which can beestablished at, for example 8 seconds, and even if there is at least oneretry left in step 165, a "no" output will result from step 165.

Such "no" result sets a "failure" flag in step 175 which normally wouldend the search as an unsuccessful search in step 183 and cause the readbeam to stop anywhere it happened to be at the time time ran out fromstep 131 and/ or the last retry left in step 165 was used up.Alternatively, if the disc contains multiple program segments in abanded arrangement with each band separated from one another and likeprogram material was also separated by a constant K (in terms of tracknumber code), then step 177 results in a "yes" output, and assuming thefirst band only had been searched to this point, at least one band wouldbe left in step 179 such that step 181 would add a constant K to thevalue T_(o) and initiate a new "start search" input to step 153. Thisprocess continues until no bands are left, i.e., the target trackincremented by the value K, 2K, 3K, etc. was not found in any band, andstep 179 outputs a "no" result to again trigger step 183 to cause theread beam to stop anywhere. Note should be made of the fact that, evenwhen all of the possibilities of retrying, incrementing and retrying,incrementing and retrying and jumping to a duplicated bands and retryingresult in failure, the "stop anywhere" step 183 may still reasonablysatisfy the user, since all of the searching in the later steps of thesearch function will naturally put the read beam in the vicinity of thetarget track, incremented target track, duplicate target track, orduplicate incremented target track.

Since the number of retries is variably set in step 153, and since,likewise, the target track can be incremented with each retry, the readbeam will ultimately land on a track adjacent to or close to the initialtarget track, and this flexibility of selecting the number of retriesand ability to increment creates a theoretical landing pad which isvariable in width depending upon the number of retries and the timeallowed for completing the search.

FIG. 6 shows pictorially the theoretical scanning motion of a read beamattempting to locate track T_(o) starting from track number zero. As thebeam initially traverses across the disc, an extremely large number oftracks will be traversed for each second of time, and the scale of FIG.6 has been adapted to illustrate the variable landing pad feature of theinvention and is not necessarily to scale. From the initial search path211, the read beam traverses path 213 searching for track T_(o). Havingpassed it, the player recognizing that the difference between the targettrack number and the current recovered track number is increasing, itreverses direction and follows path 215. Assuming that the portion ofthe path 215 is the result of one of the higher speed scanning motionsdescribed earlier, an incremented step function, at 217 also describedearlier, is used to step the beam to the target tack. Assuming that thetarget track number (same number) is not found along path portion 217,the scanning beam reverses again to follow path 219, and this procedurecontinues until a fixed period of time elapses, and in the instant case,the first try as has been just described will terminate at 4.25 seconds,and the beam passing letter A in FIG. 6 indicates that T_(o) was notfound during the first try. From position A, the scanning beam continuesto search for an additional 1.5 seconds (first retry), but this time amatch in function generator 37 is being looked for between the presentposition track number and T_(o) +1. If, the first retry time expires,i.e., after 1.5 seconds from termination of the first try, the scanningbeam performs a search starting from point B, this time searching fortrack number T_(o) +2. Not having found T_(o) +2, another retry isinitiated at letter C, and a success in locating a track number T_(o) +3is shown by the letter X in a box at point D. As explained in connectionwith the description in FIG. 5, although it is optional to do so, theinitial target track can now be arrived at by calculating the differencebetween the track number "found" at D and the original target tracknumber sought. This will effectuate a homing-in in a local search modeby incrementally stepping the read beam from point D to point E (atcircle X), and this time is relatively short in the example given,having only to have traversed three tracks to arrive at the initiallydesired track.

Single Field Step Mode

In order to understand the operation of the field step mode, it isnecessary to review the operation of the stop-motion mode of a player.

Referring to FIG. 7, there is shown a block diagram of the stop motionsubsystem employed in the videodisc player. The video signal from the FMdetector and demodulator 23 is applied to an input buffer stage 271. Theoutput signal from the buffer 271 is applied to a DC restorer 273. Thefunction of the DC restorer 273 is to set the blanking voltage at aconstant uniform level. Variations in signal recording and recoveryresult in video signals with different blanking levels. The output fromthe DC restorer 273 is applied to a white flag detector circuit 257. Thefunction of the white flag detector 257 is to identify the presence ofan all white level video signal existing during an entire line of one orboth fields contained in the vertical interval of a frame of videoinformation, although the white flag may take other forms. One such formwould be a special number stored in a line. Alternatively, the whiteflag detector can respond to the address indicia found in each videoframe for the same purpose. Other indicia can also be employed. However,the use of an all white level signal during an entire line interval inthe picture frame of information has been found to be the most usefuland reliable.

The vertical sync signal recovered from the video is applied to a delaycircuit 253. The output from the delay circuit 253 is supplied to avertical window generator 255. The function of the window generator 255is to generate an enabling signal for application to the white flagdetector 257 to coincide with that line interval in which the white flagsignal has been stored. The output signal from the generator 255 gatesthe predetermined portion of the video signal from the FM detector andgenerates an output white flag pulse whenever the white flag iscontained in the portion of the video signal under surveillance. Theoutput from the white flag detector 257 is applied to a stop motionpulse generator 265 through gate 259 and stop motion mode selector 261.The gate 259 has as a second input signal, the stop motion mode enablingsignal from the function generator 37 (FIG. 1).

The differential tracking error from the detector and demodulator 23 isapplied over line 77 (FIG. 1) to a zero crossing detector and delaycircuit 269. The function of the zero crossing detector circuit 269 isto identify when the lens crosses the mid points 91 between two adjacenttracks as shown with reference to line (a) of FIG. 3. This mid point isthe point at which the differential tracking error shown in line (b) ofFIG. 3 corresponds to the mid point 91 between adjacent tracks.

The output of the zero crossing detector and delay circuit 269 isapplied to the stop motion pulse generator 265. The stop motion pulsegenerated in the generator 265 is applied to a plurality of locations,the first of which is as a loop interrupt pulse to the trackingcontroller servo. A second output signal from the stop motion pulsegenerator 265 is applied to a stop motion compensation sequencegenerator 267. The function of the stop motion compensation sequencegenerator 267 is to generate a compensation pulse waveform forapplication to the radial tracking mirror in optical system 17 tocooperate with the actual stop motion pulse sent directly to thetracking mirror servo. The stop motion compensation pulse is thus alsosent to the tracking servo.

Briefly, the stop motion pulse to the tracking servo causes the radialtracking mirror to leave the track on which it is tracking and jump tothe next sequential track. A short time later, the radial trackingmirror receives a stop motion compensation pulse to remove the addedinertia and direct the tracking mirror into tracking the next adjacenttrack without skipping one or more tracks before selecting a track fortracking.

In order to insure the optimum cooperation between the stop motion pulsefrom the generator 265 and the stop motion compensation pulse from thegenerator 267, a loop interrupt pulse is sent to the tracking servo toremove the differential tracking error signal from being applied to thetracking error amplifiers in tracking controller 67 during the period oftime that the mirror is purposely caused to leave one track underdirection of the stop motion pulse from the generator 265 and to settleupon a next adjacent track under the direction of the stop motioncompensation pulse from the generator 267.

For a more detailed explanation of the stop motion function of a discplayer, reference is made to U.S. patent application No. 130,904 filedMar. 17, 1980 and assigned to the assignee of the present invention.

Using the arrangement described above, and assuming stop motion selector261 passes the output from white flag detector 257 to gate 259, a stopmotion effect is realized by causing the player to play two fields inrepeated fashion, the first field starting after detection of a whiteflag occurring during the vertical interval following the second field,and the second field having no white flag preceding it. Accordingly, anytime a stop motion command is given the player, the first and secondfields of a selected frame will be displayed. A frame advance command tothe player would result in the player skipping to the next frame, i.e.,skipping two fields such that first field displayed is again initiatedby a white flag during the vertical interval, following the second fieldand the second field without an initial white flag.

As explained earlier, it is often desirable to step through a videodisplay field by field, and this is not possible with the initiation ofa stop motion function being coincident with the detection of a whiteflag which occurs every other field. Accordingly, the present inventionprovides for the possibility of stepping through one field at a timeindependent of the occurrence of the white flag. Stop motion selector261, rather than taking its input from white flag detector 257 canalternatively take its input from single field step generator 263. Insuch a case, when gate 259 is enabled by a stop motion command, i.e.,when it receives the gate stop motion mode enabling signal, the nextvertical sync pulse from the tangential servo passing through delay 253will be effective in the single field step generator 263 to pass throughthe mode selector 261 and initiate stop motion of the display.Accordingly, independent of the position of the white flag in theincoming video, in the single field step selected mode, any two adjacentfields will be displayed, the single field step generator 263 ensuringthat every other vertical sync pulse from the initial one will cause thetracking servo to drive the reading beam in reverse by one track.

FIGS. 8 and 9 show, respectively, the paths taken by the reading beamalong a spiral path in the reverse and forward single field step modes.In FIG. 8, a disc portion 277, greatly exaggerated for purposes ofexplanation, has a spiral track thereon, with a portion 279 indicatingthe path of a reading beam in a normal stop motion mode. From thisdiagram, it can be appreciated that fields C and D are being traversedrepeatedly. Assuming the beam is traversing field C when a field stepreverse command is given, the arrow at 281 can be followed to illustratethat a jump back must occur at the next vertical interval and then everyother vertical interval thereafter in order to have the read beamtraverse repeatedly field C and B. It should be recognized that with anormal frame step reverse command, the fields traversed by the read beamwould be A and B, while in the single field step reverse mode, thefields being traversed are B and C.

Assuming now that the single field step command was given while thereading beam is traversing field D, the arrow at 283 can be followed toshow the need for a jump back at the next vertical interval, followed bya further jump back at the next vertical interval, and thereafterjumping back every other vertical interval. Accordingly, when thecommand is given during field D, jump back pulses must be sent to thetracking servo for each of the next two vertical interval times.

Turning now to FIG. 9, the single field step forward mode isillustrated. Again, a normal stop motion function is illustrated by theline indicating traversal of the reading beam along fields C and D in acontinuous repeating manner. This is shown at 285 in the drawing.Assuming that the jump forward command is given during the traversal offield C, arrow 287 shows that the second next vertical sync pulse shouldnot cause a step reverse, as would normally be the case in order tofollow path 285, and this will permit the read beam to continue pastthat second next vertical interval to traverse field E, after which thenext vertical sync and every other vertical sync thereafter will producerepeated playing of fields D and E. Should the command to step a singlefield forward be given during the traversal of field D, arrow 289 can befollowed to show that the next vertical sync pulse in time should beeliminated and that the next vertical sync pulse and every other syncpulse thereafter be used to effectuate a kick back of the tracking servoto again produce repeated playing of fields D and E.

A more detailed description of the circuitry and function of the singlefield step generator 263 can be found by reference to FIG. 10. Thecircuit of FIG. 10 will be explained with the aid of the waveformsdiagram of FIG. 11.

In the normal frame stop motion mode, when a stop command is given,flip-flop 261 takes a "0" set, and the Q output enables AND gate 290,while the 1 output, being a logical "0", inhibits AND 286. Theenablement of AND gate 290 permits the white flag from white flagdetector 257 (FIG. 7) on line 303 to pass through OR gate 275, andarrive at AND gate 259 which, when enabled by the gate stop motion modesignal on line 302, initiates action of stop motion pulse generator 265on line 304. In this mode, each white flag (one per frame) on line 303causes a kickback of the tracking servo to repeat the previous twofields in the normal stop motion mode.

When a field step reverse or field step forward pulse is generated, forexample manually by the user pushing an appropriate push button, a pulsepasses through OR gate 287 to set flip-flop 261 which disables AND gate290 and enables AND gate 286. Thus, the normal frame step reverse modeis defeated by the disablement of AND gate 290.

As explained in connection with FIGS. 8 and 9, the first three lines ofFIG. 11 indicate that a vertical sync pulse causes a kickback of thetracking servo for every two vertical sync pulses occurring, and thesequence of fields played is C, D, C, D, etc. If a step reverse commandoccurs during field C, FIG. 11 shows, at line 5, letter D, that an extrakickback pulse must occur at the next vertical sync time and every othervertical sync time thereafter. This is accomplished in the circuit ofFIG. 10 by the setting of extra jump back flip-flop 288 whose Q outputpasses through OR gate 284 and, when coincident with jump back timepulse on line 276 which ensures that the jump back will occur during thevertical interval and at the proper time during the vertical interval,AND gate 285 passes a true signal through AND gate 286 (previouslyenabled by flip-flop 261), through OR gate 275, and through AND gate 259to the stop motion pulse generator. Since the jump back time pulse online 276 is time related to the vertical sync pulse time, this path ofsignal flow indicates that the next vertical interval after theinitiation of the field step reverse command, i.e., at letter D, willindeed cause the tracking servo to kickback one track. The output of ANDgate 285, going true, enables AND gate 289 to reset flip-flop 288.However, the other input to AND gate 289 from the "0" side of flip-flop283, prevents resetting flip-flop 288. The "0" side of flip-flop 283 istrue, because flip-flop 283 has been set by AND gate 282 by the verticalsync pulse on line 280 which, having set flip-flop 281, allows jump backflip-flop 283 to toggle to its true state. Thus, after initiation of thefield step reverse command during field C, OR gate 284 receives a truesignal on both inputs to effect the kickback at the next vertical synctime. This can be observed by reference to the fifth line of FIG. 11,letter D. The next vertical sync pulse in time (at E), however, does notkick the tracking servo, since the first vertical sync pulse toggledflip-flop 283 to the false state, and this enables AND gate 289 to resetflip-flop 288 such that both inputs to OR gate 284 are false, and thusthe second vertical sync pulse has no effect on the tracking servo. Thismay be observed by reference to the fifth line of FIG. 11 at letter E.

Having reset flip-flop 288, it will not be set again until the nextfield step reverse command by the user on line 278a is given. However,since flip-flop 283 is a toggle flip-flop, the next vertical sync pulsein time, occurring at letter F in FIG. 11, will set flip-flop 283 to thetrue state, and the Q output of flip-flop 283 passes through OR gate 284to again create a stop motion pulse. The continued toggling action offlip-flop 283 for each vertical sync pulse will ensure that, from thispoint on, every other vertical sync pulse will produce a stop motionpulse, and the objective set forth in connection with the description ofFIG. 8 for a step reverse pulse occurring during field C will have beenaccomplished. Line 6 of FIG. 11 then shows the resultant order of fieldsbeing displayed, and it can be seen that after the field step reversecommand, fields B, C, B, C, B, etc. are being displayed, as is desired.

In the event that the field step reverse command occurs during field D(see line 7 of FIG. 11), the 0 side of toggle flip-flop 283 will be trueenabling AND gate 289. At the next vertical sync time (at C), toggleflip-flop 283 will flip to its true state, and through OR gate 284 astop motion pulse will be generated at jump back time in the mannerpreviously described. At the same time, the output of AND gate 285 willbe sent to the second input of AND gate 289, and as a result flip-flop288 will be reset, as well as toggle flip-flop 283. Thus, althoughflip-flop 283 was reset by toggle action on the previous vertical syncpulse, it is now reset again on the next vertical sync pulse through thepath just described, i.e., through OR gate 284, AND gate 285, and ANDgate 289. Thus, when a subsequent vertical sync pulse occurs, at letterD in FIG. 11, another stop motion pulse will be generated due to toggleflip-flop 283 going true again. As a result, two kick back pulses in arow, at times C and D in FIG. 11, will have occurred to produce thesequence of fields being played as shown in line 9 of FIG. 11.

To accomplish a field step forward function, it is only necessary thatthe effect of the next vertical sync pulse in time be eliminated,therefore producing a sequence changing from the recycling of fields Cand D to the recycling of fields D and E, and this is true whether thefield step forward command occurred during the playing of field C orfield D. To accomplish the field step forward function, an "inhibit onecycle" flip-flop 281 is employed. When the field step forward command isreceived over line 278b, flip-flop 281 is reset to the 0 state, and theQ output goes low to inhibit AND gate 282 and AND gate 285, therebyprecluding passage of the next vertical sync pulse in time (at D forlines 10-12, and at C for lines 13-15 of FIG. 11) to the jump backtoggle flip-flop 283, and to preclude the next jump back time pulse online 276 to create a stop motion pulse. However, the second nextvertical sync pulse in time (at E and D, respectively) will again setflip-flop 281, and its output going true will again enable gate 282 topass the vertical sync pulses occurring thereafter, will enable AND gate285, and permit stop-motion action to resume. Accordingly, afterelimination of one vertical sync pulse for each field step forwardcommand, the toggle flip-flop 283 proceeds to produce a kickback of thetracking servo for every other vertical sync pulse in the mannerdescribed earlier. The waveforms of lines 10 through 15 of FIG. 11should be, therefore, self-explanatory.

Random Command Function

FIG. 12 shows a functional block diagram of the random command functionaccording to the present invention. FIG. 13 illustrates graphically themotion of the read beam when following a series of random commandinstructions.

When a disc is being played, each operational function of the machineunder control of the microprocessor begins in orderly fashion after theconclusion of the preceding instruction. As a result, the orderlysequence of instructions can be thought of as an instruction streamflowing from the microprocessor and being interpreted by the player byanalyzing the commands contained in each instruction. FIG. 12illustrates the basic operation of receiving an instruction stream overline 297 and extracting, in command extractor 291, the command portionof the instruction and routing it to command interpreter 292 which thentranslates the command instruction to functional commands for theplayer. Likewise, the instruction stream enters argument extractor 293,and the numbers converter 294 extracts the argument portion of theinstruction and sets it in shift register 295. The argument, for examplea particular frame number in the recorded video program, is sent overline 300 to command interpreter 292 and becomes a part of the commandfunction. Thus, the command output of command interpreter 292 contains aplayer functional control portion and an argument portion. Generally,the argument portion of the instruction sets forth a particular frame,and the command portion tells the player what to do at or with thatframe. In a typical instruction, for example, the instruction "1200search" is interpreted as commanding the player to carry out itssearching function while looking for frame number 1200.

As explained earlier, in instances where the argument portion of theinstruction is advantageously an unpredictable or random number, it isnecessary that the argument extractor 293 not pass the argument throughthe command interpreter directly as received from the instructionstream, but rather to supply a random number on line 300.

To accomplish this desired result, a command, for example RND command,recognized by the command extractor 291 is sent over line 298 to a shiftregister 295 in the argument extractor 293. This recognition of the RNDcommand will load a random number from random number generator 296 intoshift register 295. As a result, a random number is outputted over line300 as an argument to the command interpreter, and the stated objectiveis met.

If desired, the normal argument in the instruction converted in numberconverter 294 can be used as a basis for establishing a reference forthe random number generator 296. For example, rather than to have anyrandom number produced by generator 296 to be loaded into shift register295, the number exiting numbers converter 294 can represent a startingnumber from which a set of random numbers takes reference. Thus, if theargument exiting numbers converter 294 is 1200, the numbers loaded intoshift register 295 could be any selectable range of numbers beginning at1200. If a quiz having ten questions is presented at frames 1200 through1209, then random numbers generator 296 would merely add, for eachrandom command, any one of numbers 0 through 9 to the 1200 numberexiting numbers converter 294, and this would result in loading shiftregister 295 with one of the numbers randomly selected from the set ofnumbers 1200 through 1209.

After the command instruction has been executed, a command end signalexiting the command interpreter 292 over line 299 will clear shiftregister 295 ready for the next instruction. In FIG. 13, the line 301indicates a linear relationship between playing time and the number ofthe track being played at that time. At some point in time, for exampleafter perhaps 12,000 tracks have been played, a random commandinstruction could be given to instruct the player to jump to any one ofa plurality of tracks in the vicinity of track 25,000. This couldhappen, for example, if an "autostop" command were initially set at thebeginning of playing time in FIG. 13, the "autostop" command merelyplacing the player in a stop motion condition upon recognition of acomparison between a preset frame number and the frame number recoveredfrom the disc while playing. Thus, if the autostop frame number had beenchosen at frame 12,000, the player would play along line 301 in FIG. 13until frame 12,000 was identified in the comparator, and reference ismade to the previous discussion of FIG. 1 concerning blocks 29, 30, and37 of that figure.

At the conclusion of that "autostop" command, the next instruction isassumed to be a random command instruction causing the player toimmediately search for a randomly selected one of a plurality of numbersat track 25,000. This is shown at point A in FIG. 13. After arriving atthe randomly chosen frame number in the vicinity of track 25,000, theplayer plays that randomly selected track shown at 302 in the drawinguntil another command instruction is received. In FIG. 13, twopossibilities are shown, one at point B merely being a repetition of thekind of random search instruction given at point A, with the exceptionthat after finding the randomly selected track in the vicinity of track45,000, the randomly selected track would be played in a stop motionmode indicated at 303 in the figure. Had the random number selected beena different one, it could have followed a different command at the endof playing that segment of the disc identified in the figure as 302, andas a result, a search command could have directed the reading beam tothe vicinity of track 20,000 to stop motion at one of an arbitrarily orrandomly chosen track in that vicinity, and this is shown at 304 in thedrawing. At point D is illustrated the possibility of returning, againrandomly, to one of the tracks in the vicinity of track 25,000 in amanner similar to that described in connection with point A in thefigure. If the group of tracks at point A were, for example, questionsto a quiz, after having passed through points D and/or C, another one ofthe same group of questions could be presented at point D, and the playmode shown at 305 would be similar to that shown at 302, and the processrepeated until all of the questions of the quiz, or a predeterminednumber of them, were asked. In this connection, random number generator296 can be, if desired, arranged to randomly select a number from agroup of numbers without repeating a prior selected number. This wouldassure that the same question would not be asked twice on a quiz.

Of course, other applications of the random command function describedabove are possible than those associated with presenting questions to aquiz, and it should be understood that the invention is not to be sonarrowly interpreted as to be limited to the specific examples presentedabove.

Multiple Track Jump Command

Having described the manner in which the tracking controller 67 operatesin FIGS. 1 and 2 and the manner in which the stop motion mode functionsaccording to FIG. 7 and its associated description, the presentinvention also includes a method and means for combining these twofunctions to produce certain special effects heretofore not realizable.Furthermore, with the additional capabilities of the player having afield step mode, described earlier, additional benefits and functions ofthe player can be uniquely produced.

The multiple track jump function of the player is to be distinguishedfrom the jump function associated with a search mode operation. In thesearch mode of operation outlined in the aforementioned U.S. patentapplication No. 310,021, after a carriage movement has brought thereading beam to within a prescribed distance from a target track, a jumpcommand is given to the tracking mirror in the optical system 17 of theinvention to effectuate a broad jumping of tracks to arrive at thetarget track, counting the number of tracks jumped while the read beamis on its way to the target track. However, in a search mode of thisnature, the number of tracks to be jumped are usually substantial, and anumber of jumps are accomplished during each half revolution of thedisc. Jumping tracks in this manner produces severe visual noise in thereproduced picture, since the multiple track jumping occurs at anarbitrary circumferential location on a disc such that any jumpingoccurring outside the vertical blanking interval will be seen as noiseon the screen.

In the instant case, the number of tracks jumped are to be held to avery small number, on the order of ten or less, and with improvedelectromechanical devices for the tracking servo, jumps of up to tentracks can be made during the vertical interval blanking time withoutdisturbing visual noise being generated. Furthermore, as opposed to asearch-type of track jumping where a present track address is subtractedfrom a target track address and the number of track crossings arecounted to permit landing of the reading beam at the target track, inthe present invention the multiple track jumping is to be effected atregular intervals or according to a prescribed pattern with a playfunction occurring between each jump function.

This is exemplified in FIG. 14 by way of example. In FIG. 14a, theletters represent a particular visual picture frame as shown in FIG.14b. The left to right direction of FIG. 14a represent a spatialarrangement of the individual picture frames A through D. With theparticular arrangement shown, starting with frame 311, picture frame 323would be shown first on the screen. The loop 313 over letter C suggeststhat an adjacent track containing picture C is jumped over by kickingthe tracking servo, in the manner previously described, so as to playtrack B which will result in a display of picture frame 325. Similarly,the next picture frame B will be jumped over, and frame C will bedisplayed as shown at 327, followed by frame D shown at 329. Afterplaying track D, loop 315 represents a jump back of two frames to playframe 317 which again contains the information of picture frame A shownat 323. This reverse looping, jumping back two tracks and playing one,continues until the last loop 319 leads to the playing of track D shownat 321 which then continue in a play mode to again play track A at 311and the entire process repeats. In this manner, the sequence of pictureframes displayed on a monitor would be A B C D A B C D A B C D, etc. Inthe simplified pictures illustrated in FIG. 14b, the effect of thiscontinual repetition of the four picture frames in sequence will be thatof a moving marquis shown at 331. Of course, more frames can beinterleaved, and other more complicated figures can be contained in thetracks so as to give motion or other special effects to the recordedmaterial. The advantage of such a system should be readily apparent,i.e., in the example shown a continuous motion of indicia across ascreen is accomplished by utilizing only eight tracks of the videodisc,whereas in the past, motion presentation on the screen required realtime territory for the same kind of presentation, i.e., 30 frames foreach second of time the effect is to be displayed.

Since the time in which the tracking servo is kicked forwards orbackwards is under microprocessor control of the function generator 37,if the example of FIG. 14b is to show the indicia moving at a slowerrate, each track played in the example given could be played 2, 3, ormore times before jumping to the next track.

Another extension of the multiple track jump function can advantageouslyproduce forward or reverse multiple playing speeds. FIG. 15 shows inblock diagram form a preferred arrangement for accomplishing multipleplaying speeds by utilizing the multiple track jumping effect. Again, itis to be realized that all jumping of tracks occur during the verticalinterval, and therefore the circuit of FIG. 15 produces completelysynchronous picture display at multiple speeds, forward or reverse,without the distraction of visual noise as in prior art track jumpingdevices.

In analyzing FIG. 15, it will also be appreciated that, with theadditional flexibility of being able to jump field by field in themanner described earlier, other advantages in simplifying theelectronics can be realized. Accordingly, the circuit of FIG. 15 willutilize either a frame jump or a field jump mode, depending upon whichis more suitable for the playing speed desired.

Similar to the jumps down counter 101 in FIG. 2, a jumps counter 519 isshown in FIG. 15 which is loaded with a number N on line 520representing the number of tracks to jump on command. Also, similar tothe decrementing of counter 101 by the zero crossing detector 99 of FIG.2, a zero crossing pulse to decrement jumps counter 519 is inputted online 521.

Kick generator 516 will impart a radial motion to the tracking mirror,forward or backward, depending upon whether the microprocessor issending a forward signal on line 507 or a reverse signal on line 506.Kick generator 516 is responsive to a true output from jump flip-flop515 each time it is set. Either AND gate 517 or AND gate 513 can setflip-flop 515 and effectuate a kick to the tracking mirror if enabled byjump enable signal on line 504. Jump enable occurs when a jump of one ormore tracks is called for by the microprocessor.

Since advantage can be taken of jumping every field, as opposed to everyframe, an "every field enable" signal on line 505 is another input toAND gate 517, and through inverter 518 it disables AND gate 513.Accordingly, when every field is to produce a jump, AND gate 517 willpass vertical sync pulses from line 511, and when a jump is to occuronce per frame, the "every field enable" signal on line 505 will be lowdisabling AND gate 517 and, through inverter 518, enable AND gate 513.Flip-flop 512 is a toggle flip-flop which produces a true output to ANDgate 513 for every other vertical sync pulse received on line 511. Thus,when a jump is to be effected, jump enable line 504 is brought true, andeither every vertical sync pulse from AND gate 517 or every othervertical sync pulse from AND gate 513 will pass through OR gate 514 andset jump flip-flop 515 to create the kick to the tracking mirror. As thetracking mirror moves across each track, during the vertical interval,each zero crossing decrements counter 519 until the number of tracks tobe jumped has been counted down to zero in counter 519, at which time areset pulse from counter 519 resets jump flip-flop 515 so as to precludeany further kicking motion of the tracking mirror.

Since kick generator 516 will react only if a number is loaded intojumps counter 519, if no number is loaded, no stop motion pulse isgenerated, and the player plays forward at normal speed. Moreover, whenthe player is in the normal "play" mode, no jumps will be made, sincethe read beam follows a spiral track. A table, FIG. 16, shows theconditions for jump direction and for the value of N or M, i.e. thenumber loaded into jumps counter 519, for a variety of play speeds, bothforward and reverse, as well as stop. The table shows that no jumps arerequired in the play forward mode.

The number of tracks to jump is different if the kick generator issubject to being pulsed at a field rate than if it is being pulsed by aframe rate. Accordingly, if "EVERY FIELD ENABLE" line 505 in FIG. 15 isa logical "1", then jumps counter 519 is loaded with a number Mdesignating the number of tracks to jump each field change, while if"EVERY FIELD ENABLE" line 505 is a logical "0", jumps counter 519 isloaded with a number N designating the number of tracks to be jumpedeach frame change. Thus, N or M takes on different values depending uponwhether the "every field enable" signal on line 505 is a logical "1" orlogical "0". If the "every field enable" signal is a "0", then when thenumber of jumps equal 1 and the jump direction is forward, the playerplays at a times two, (2×), speed. Since one revolution of the disccauses the read beam to follow one spiral track forward, in order to geta normal play reverse speed, the kick generator must be pulsed inreverse, and the number of tracks to jump per revolution is 2.Continuing the analysis along these lines produces the numbers in theright hand column of FIG. 16, and it can be appreciated that byselecting the value of N, any play speed which is a multiple of thenormal play speed can be selected by the various values of N given. Itshould be borne in mind, however, that, as stated earlier, since thejumps must occur in the vertical interval to avoid visual noise, thereis a practical limit to the upper values of N in either forward orreverse direction.

With the capability of jumping during the vertical interval on a fieldbasis, the number of tracks jumped to produce higher values of playspeed will be less, and thus a more reliable, noise-free picture can beplayed at higher play speed than with the limitation of being able tojump only on a per frame basis. The second column of FIG. 16 illustratesthis fact by showing that when jumping forward one track every field,i.e., when M=1, a 3× speed can be realized, and jumping two tracks eachfield, (M=2), produces a 5× speed in the forward direction. In anextension of this analysis in the reverse direction, normal play reversespeed can be produced by jumping back one frame every field. Similarly,a -3× speed is produced with a jump of two tracks per field in thereverse direction, while a -5× speed occurs with a reverse jump of threetracks per field. Of interest in the field step mode is the fact thatcertain play speeds are not possible, for example a 4× forward, a 2×forward, a 2× reverse, and a 4× reverse speed.

In a general analysis, again realizing the limitations on the number oftracks that can be jumped during the vertical interval, a forward speedof (N+1) X is produced with N jump forward for each frame, while a (1-N)X speed results in the reverse frame jump mode. Similarly, with M jumpsevery field, forward speeds will calculate to (2M+1)X, while reversespeeds will calculate to (1-2M) X.

White Flag Wait Function

It is often desirable to transmit information contained on a videodiscto an external using device. The device may be asynchronous with selfclocking capabilities such that it is only necessary to transmit theinformation to the external device when it calls for it or when thetransmitting device sends a precursor pulse to prepare the externalequipment for the information to be transmitted immediately followingsuch precursor.

The procedure for transmitting information from a videodisc wouldnormally be to play or search to a certain frame number, perform anautostop at that frame number (i.e., enter a stop motion mode), andsubsequently enter into a play mode such that at the start of the nextframe, the vertical sync pulse will trigger transmission of theinformation to be transmitted to the external device. A problem arises,however, due to the fact that an autostop function replays two fields,it is not always predictable where the read beam will land in theautostop mode. As a result, the next vertical sync pulse could be at theend of the first field or at the end of the second field of the framestopped on. Since the external equipment must not receive falseinformation from the incorrect field, it is necessary to reliablyestablish which field is being played, so that transmission after secondfield (assuming that is the desire of the user) will be effectuated.

FIG. 17 illustrates the waveforms needed to perform transmission of thedata at the proper time. A vertical sync pulse on line 611 occursbetween fields, as is known, and this is represented in FIG. 17 byequally spaced pulses. During alternate vertical intervals, a white flagsignal 613 can be detected as was described in connection with whiteflag detector 257 in FIG. 7. The data to be transmitted is shown on line651 to begin at point B and lasts for four fields. The data to betransmitted may be video in nature, pulse code modulated audio, analogaudio, or digital data on either the audio or video channels of therecorded program.

Stop motion is accomplished in the manner described in connection withFIG. 7 and is shown on line 617 of FIG. 17 in the form of still framingbetween points A and B.

For proper and effective transmission of the data on line 615, atransmit enable signal 619 must begin on the vertical sync pulse justprior to point B so as to give advanced indication to the external unitthat data flow is about to start. A data flow gate 621 is then generatedat the next vertical sync pulse (time B) and lasts for any preferredduration, and in the example given here for a period of four fields.

FIG. 18 is a block diagram employed to implement the white flag waitfunction described in connection with the waveform analysis of FIG. 17.Recalling again from FIG. 1 the searching feature of the functiongenerator 37, address recovery 29, and register 30, a search algorithm629 is performed in the manner described in the aforementioned U.S.patent application No. 316,021 having received a target frame number online 637 and a search enable command on 639. Assuming the search hasbeen successful to find the target frame number, an output on line 643causes the autostop algorithm 631 to perform its function of stop motionat the frame preceding the data to be transmitted. At the conclusion ofthe auto stop, i.e., after successful location and commencement of thestill framing mode at the target track, a play command is routed to thetracking servo over line 633 to begin the play mode operation of theplayer.

From the video in line 635, white flag detector 257 extracts the whiteflag signal and outputs it over line 613 to white flag wait comparator627. Thus, during the play mode of an autostop function, the next whiteflag will generate an output of comparator 627 over line 641, and thisoutput is routed to the transmit enable pulse generator 623.

In the autostop mode, it can be appreciated by reference to FIG. 17 thatduring the play portion of the still framing function, only the whiteflag immediately following point B will be detected since the read beamis kicked back from point B to point A due to the detection of the whiteflag occurring immediately following point B. Since white flags, as wellas other vertical and horizontal timing signals are arranged radially ofthe disc, a jump back caused by a white flag at point B will place theread beam downstream of the white flag at point A. Thus, the signal fromcomparator 27 on line 641 must necessarily be the white flag followingthe vertical sync at point B. As a result, transmit enable pulsegenerator 623 is enabled by that white flag occurring immediately afterpoint B, and, after kickback to point A, the next vertical sync pulse online 611 produces an output on line 619 which begins at the nextvertical sync pulse in time, and through the use of appropriate andknown toggle flip-flop and gating, the transmit enable signal 619 willbe terminated on the second next vertical sync pulse. As a result, atransmit enable pulse in the form of a window of time has been generatedto span the vertical pulse occurring at point B. The transmit enablesignal, together with the next vertical sync pulse in time from line 611will create a data flow gate in block 625 which gates the data to betransmitted over line 615 to the external equipment. Again, dependingupon the amount of time needed for transmission of the data, appropriatecounters and gating can be arranged in a known manner to terminate thedata flow pulse 621 at the proper time. In the instant case, a counterand gate structure allowing four fields of data to be transmitted isshown.

It can thus be seen that no matter what field the read beam lands in inthe auto stop mode, initiation of the transmit enable pulse will nottake place until after a white flag occurs during the normal playportion of the autostop function, and this "white flag wait" functioninginsures proper transmission of the data coincident with the verticalsync pulse occurring at point B in FIG. 17.

Although the invention has been described in detail with reference toits presently preferred embodiments, it will be understood by those ofordinary skill in the art that various modifications can be made,without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited, except asby the appended claims.

I claim:
 1. A method for manipulating a track reader for recoveringinformation from a selected target track of a plurality of spirallingand substantially circular information tracks or concentric circularinformation tracks on a record disc, each said track containing at leastin part a unique track identifying code, said method comprising thesteps of:(a) generating a target track identifying code that matches thetrack identifying code of said target track; (b) rotating the disc in aprescribed fashion; (c) scanning the disc with the track reader at aprescribed radial velocity relative to the disc, toward the target trackto recover information recorded thereon including track identifyingcodes; (d) comparing recovered track identifying codes with said targettrack identifying code and detecting the difference therebetween; (e)scanning continuously toward said target track until said comparing anddetecting step indicates that the recovered track identifying code iswithin a fixed range of the target track identified code; (f) thereaftermoving the track reader incrementally within said range relative to saiddisc across a prescribed number of tracks during each revolution of thedisc, toward the target track; (g) upon passing the target track bydetection in said comparing and detection step, that the differencebetween the recovered track identifying code and the target trackidentifying code is increasing, reversing the radial scanning direction;(h) continuing said incremental scanning while continuing to detect thedifference between the recovered track identifying code and the targettrack identifying code, and continuing to reverse the radial directionof movement upon each detection of an increasing difference between therecovered track identifying code and the target track identifying codeuntil a match is made therebetween, and in the absence of a match in afirst prescribed period of time, incrementing the target trackidentifying code by a fixed value; (i) continuing said incrementalscanning toward said target track until a match between the recoveredtrack identifying code and the incremented target track identifying codeis found; and (j) after a match is found, ceasing the relative radialmovement across the disc and recovering the information from the arrivedat track.
 2. The method as claimed in claim 1, including, in the absenceof a match in step (i) and before step (j), the further steps of:(k)detecting the difference between the recovered track identifying codeand the incremented target track identifying code and continuing toreverse the radial direction of movement upon each detection of anincreasing difference between the recovered track identifying code andthe incremented target track identifying code until a match is madetherebetween, and in the absence of a match in a further prescribed timeperiod, incrementing the incremented target track identifying code by apreset value; and (l) continuing said incremental scanning toward saidtarget track until a match between the recovered track identifying codeand the further incremented target track is found.
 3. The method asclaimed in claim 2, including, before step (i), the step of:repeatingstep (j) until a fixed time interval has elapsed from the initiation ofstep (b); and wherein step (i) includes, after a match is found or aftersaid fixed time interval has elapsed, whichever is first, ceasing therelative radial movement across the disc and recovering the informationfrom the arrived at track.
 4. The method as claimed in claim 1, whereinstep (j) includes, after a match is found and before the recovering ofinformation from the track arrived at, the steps of:comparing the lastincremented target track identifying code with the first of said targettrack identifying codes; and moving the track reader toward said targettrack across a number of tracks equal to the difference between saidlast incremented target track identifying code and said first targettrack identifying code.
 5. The method as claimed in claim 1, wherein therecord disc contains N duplicate programs on the same disc side, where Nis at least two, and wherein like tracks of each program are separatedby K tracks, and wherein step (j) includes, after said fixed timeinterval has elapsed and a match has not been found, and before saidceasing and recovery steps are performed, the steps of:adding the valueK to the first-mentioned target track identifying code to produce anupdated track identifying code; scanning continuously toward saidupdated target track until said comparing step indicates that therecovered track identifying code is within a fixed range of the updatedtarget track identifying code; and scanning incrementally across aprescribed number of tracks during each revolution of the disc, towardthe updated target track.
 6. The method as claimed in claim 5 includingthe steps of:upon passing the updated target track by detecting that thedifference between the recovered track identifying code and the updatedtarget track identifying code is increasing, the step of reversing theradial scanning direction; and repeating the functions of steps (h),(i), and (j), replacing the target track and target track identifyingcode with said updated target track and updated target track identifyingcode values, respectively.
 7. Apparatus for recovering information froma selected target track of a plurality of spiralling and substantiallycircular information tracks or concentric circular information tracks ona record disc, each said track containing at least in part a uniquetrack identifying code, said apparatus comprising:means for generating atarget track identifying code that matches the track identifying code ofsaid target track; means for rotating the disc in a prescribed fashion;a track reader for scanning the rotating disc to recover informationrecorded thereon; coarse positioning means for moving the track readerat a prescribed radial velocity relative to the disc, toward the targettrack; means for comparing recovered track identifying codes with saidtarget track identifying code; means for detecting the differencebetween said recovered track identifying codes and said target trackidentifying code; fine positioning means, responsive to said detectingmeans and operable after the coarse positioning means has moved thetrack reader into within a number L of tracks from the target track, formoving the track reader incrementally across a prescribed number oftracks during each revolution of the disc, toward the target track;means for reversing the radial direction of radial scanning movementupon said detecting means detecting that the difference between therecovered track identifying code and the target track identifying codeis increasing, said means for reversing the radial direction of scanningmovement continuing to reverse the radial direction of scanning movementupon each detection of an increasing difference between the recoveredtrack identifying code and the target track identifying code until amatch is made therebetween by said comparing means; means forincrementing the target track identifying code by a fixed value in theabsence of a match between the recovered track identifying code and thetarget track identifying code in a first prescribed period of time; saidfine positioning means continuing to move said track readerincrementally across said prescribed number of tracks during eachrevolution of the disc toward said target track, whereupon said meansfor comparing compares recovered track identifying codes with theincremented target track identifying code until a match is madetherebetween; said apparatus including terminating means for ceasingsaid relative radial movement when a match is made, said track readeroperable to then recover the information recorded on the track whoseidentifying code matches the incremented target track identifying code.8. The apparatus as claimed in claim 7, wherein said means for detectingis operative to detect the difference between the recovered trackidentifying code and the incremented track identifying code while saidfine positioning means continues to reverse the direction of radialmovement of the track reader upon each detection of an increasingdifference between the recovered track identifying code and theincremented target track identifying code, until said match is made. 9.The apparatus as claimed in claim 8 wherein:said comparing meanscompares the last incremented target track identifying code with thefirst of said target track identifying codes after a match is found andbefore said track reader recovers information from the track arrived at;and said fine positioning means includes means responsive to saiddetecting means for moving the track reader towards said target trackacross a number of tracks equal to the difference between said lasttarget track identifying code and said first target track identifyingcode.
 10. The apparatus as claimed in claim 7, wherein:said detectingmeans includes means for continuously detecting the difference betweenthe recovered track identifying code and the incremented target trackidentifying code; said direction reversing means includes means forcontinuing to reverse the radial direction of scanning movement uponeach detection of an increasing difference between the recovered trackidentifying code and the incremented target track identifying code untila match is made therebetween; said incrementing means includes means, inthe absence of a match in a further prescribed time period, forincrementing the incremented target track identifying code by a presetvalue; and said comparing means includes means for continuing comparingsaid recovered track identifying codes with each incremented trackidentifying code, and said incrementing means includes means forcontinuing incrementing the target track identifying code until a fixedtime interval has elapsed from the initial moving of the track reader bysaid coarse positioning means.